1. Field of the Invention
The present invention relates to an image recording apparatus such as a color electrophotographic printer that prints a color image on a recording medium, and more particularly to an image recording apparatus in which the positions of images of respective colors are corrected in order to print images of the respective colors at accurate locations.
2. Description of the Related Art
A color image recording apparatus such as a color electrophotographic printer incorporates a plurality of print engines. A tandem type color image recording apparatus incorporates four such print engines aligned in line and is capable of printing four color images on a sheet of paper statically attracted to a transport belt. The yellow, magenta, cyan, and black images are printed by successive printing operations in superposition on the sheet of paper. Thus, the tandem type color image recording apparatus achieves high speed printing.
With a print engine having an image recording element such as a miniaturized LED head, a fixedly mounted line head may be combined with an image drum unit, thereby miniaturizing the overall apparatus.
However, the respective recording heads may not be accurately positioned with respect to corresponding image drum units due to dimensional errors of the subassembly units and mounting errors encountered when the recording heads are mounted to the recording apparatus. As a result, the images of the respective colors are not accurately printed with respect to one another, causing color shift to occur.
If the mounting position of the recording head is shifted in a traverse direction in which the recording elements (e.g., LEDs) are aligned, color shift occurs in that direction. If the mounting position of the recording head is shifted in an advance direction in which the transport belt runs, color shift occurs in the advance direction. Color shift also occurs in a direction oblique to the traverse direction if the respective lines of recording elements are oblique with respect to the traverse direction.
In other words, deviations of mounting positions of the print engines in the advance, traverse, and oblique directions cause the deviations of the positions of printed images, thereby resulting in color shift. The degree of color shift varies with time for various reasons, and causes degradation of color print over time.
The present invention was made in view of the aforementioned problems.
An object of the invention is to provide an image recording apparatus in which color shifts among images of the respective colors are detected and the color shifts are corrected for stable printing operation of the color images.
An image recording apparatus includes a plurality of print engines that print images of different colors in registration to form a color image. The print engines are aligned in an advance direction in which a recording medium is transported. A transporting member transports a recording medium in the advance direction. Each of the plurality of print engines records an image of a corresponding one of different colors onto a recording medium. The print engine also prints a detection pattern of the corresponding one of different colors on the transporting member. A color shift detecting section detects an amount of color shift based on two detection patterns recorded on the transporting member. A correcting section corrects a position of the image recorded on the recording medium, the position being corrected in accordance with the amount of color shift.
A first one of the plurality of image-forming sections records a first detection pattern and a second one of the plurality of image-forming sections records a second detection pattern on the first detection pattern. The color shift detecting section detects the amount of color shift based on the first and second detection patterns.
The color shift detecting section detects the amount of color shift by measuring an intensity of light reflected by the detection pattern.
The color shift detecting section detects the amount of color shift of a corresponding color based on the detection pattern recorded on the transporting member. The detection pattern may be one of a first detection pattern for detecting color shift in the advance direction, a second detection pattern for detecting color shift in the traverse direction perpendicular to the advanced direction, and a third detection pattern for detecting color shift in an oblique direction at an angle with the traverse direction.
The detection pattern includes a plurality of blocks aligned in a row, each of the plurality of blocks having at least one stripe. A first image-forming section records a first detection pattern and a second image-forming section records a second detection pattern on the transporting member. The first detection pattern has the plurality of blocks each of which is shifted with respect to adjacent blocks by a first predetermined distance in a predetermined direction. The second detection pattern has the plurality of blocks each of which is shifted with respect to adjacent blocks by a second predetermined distance in the predetermined direction. The first detection pattern and the second detection pattern are recorded one over the other.
The color shift detecting section includes a pair of detectors that measure an intensity of light reflected back from the third detection pattern, and a circuit that selectively receives outputs of the pair of detectors.
The third detection pattern may be aligned in two rows in a staggered configuration such that each of the two rows is at an end portion of the transporting member in the traverse direction.
The detection pattern may be one of a plurality of detection patterns that have a plurality of levels of resolution. The color shift detecting section detects the amounts of color shift based on the plurality of detection patterns. The correcting section corrects a position of the image recorded on the recording medium in accordance with the amounts of color shift, the position being first corrected based on a low level of resolution and then based on a high level resolution.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.